Understanding the role of head size and neck length in micromotion generation at the taper junction in total hip arthroplasty

Modular hip implants allow intra-operative adjustments for patient-specific customization and targeted replacement of damaged elements without full implant extraction. However, challenges arise from relative micromotions between components, potentially leading to implant failure due to cytotoxic metal debris. In this study magnitude and directions of micromotions at the taper junction were estimated, aiming to understand the effect of variations in head size and neck length. Starting from a reference configuration adhering to the 12/14 taper standard, six additional implant configurations were generated by varying the head size and/or neck length. A musculoskeletal multibody model of a prothesized lower limb was developed to estimate hip contact force and location during a normal walking task. Following the implant assembly, the multibody-derived loads were imposed as boundary conditions in a finite element analysis to compute the taper junction micromotions as the relative slip between the contacting surfaces. Results highlighted the L-size head as the most critical configuration, indicating a 2.81 μm relative slip at the mid-stance phase. The proposed approach enables the investigation of geometric variations in implants under accurate load conditions, providing valuable insights for designing less risky prostheses and informing clinical decision-making processes.


Mesh convergence analysis
In the mesh convergence analysis, the element size in the contact region was progressively reduced.Starting from a dimension of 0.8 mm, the mesh size was gradually reduced until reaching 0.15 mm.The criterion for achieving mesh independence was considered met when the difference between the solutions of two consecutive mesh refinements was less than 2% in terms of both longitudinal displacement of the head (Δ) and average contact pressure of the trunnion (p̄), as illustrated in Figure S2.As a result of the mesh convergence study, an element size of 0.2 mm was adopted in the contact region of the taper junction.
The two parameters analyzed in the mesh convergence were analytically verified 2 at the end of the assembly load, obtaining the following values: -average contact pressure on the trunnion surface (p̄) equal to 24.01 MPa; -longitudinal displacement of the head (Δ) equal to 34.54 µm.

Comparative FE model
To investigate the impact of the load application method, a comparative model was also created by applying the loads at the center of the head and transmitted rigidly to its outer surface [3][4][5][6] (Figure S3).In this case, a single implicit static simulation of the same duration as the MB simulation was conducted taking into account the MB-derived reaction torques measured at the head-neck fixed joint (Figure S4).Under this boundary condition, the CSLIP peak calculated for the reference configuration at t2 decreases by approximately 20% compared to the model where loads are applied to the head surface, thereby underestimating micromotions at the taper junction (Figure S5).    and in silico 9 findings reported in the literature.

Figure S2 .
Figure S2.Outcomes of the mesh convergence analysis of the analytically verified parameters: a) longitudinal displacement of the head (Δ) and b) average contact pressure of the trunnion (p̄).

Figure S3 .
Figure S3.Comparative FE models are depicted for the reference configuration: a) Application of the MB-derived load in six time steps on the head surface.b) Application of the MB-derived reaction forces and torques at the head center.The distal end of the trunnion was fixed (black triangle) in both models, while yellow lines represent rigid links.

Figure S4 .
Figure S4.Components of the multibody-derived reaction torques measured at the head-neck fixed joint by varying the head size and maintaining the same implant offset.

Figure S5 .
Figure S5.Comparison of the Maximum Relative Contact Slip (CSLIP) for the reference configuration obtained for the two FE models.The solid line represents the results obtained by applying loads derived from MB on the head surface, while the dashed line represents the results obtained by applying MB-derived loads at the center of the head.

Table S1 .
Muscle bundle included in the multibody model with the relative maximum isometric force derived by the 'Gait2392' model provided by the OpenSim software 1 .

Table S2 .
Cartesian coordinates (mm) of the contact points on the femoral head at the considered instants (t1-t6) by varying the implant offset.
Hip contact force (HCF) validationFigure S6.Comparison of the resultant HCF computed by means of the multibody model against in vivo